Yes, of course SK was originally devised as a tool for learning ASL. It is not clear, at least to me, whether it is considered a separate product line. There is some support from MMP, such as Basic Training in 2025 and SKEP 3 in 2023; however, a total of only 115 official scenarios and a single CG released in 20 years does not suggest strong commitment.
If it is really considered or defined as a separate product line is not really of consequence in the end. It more matters that it factually works like one IMHO.
I have not followed how many SK scenarios are out there or if besides those by MMP others (or how many) come from third party publishers. In any case, it will take you a good while to play through roughly 120 scenarios and the campaign games "Decision at Elst" and upcoming "Marco Polo Bridge". And truth be told - if you have done all this, you are ready for ASL.
While I am pretty sure that SK was not planned as a seperate product line but rather evolved into one also needs to be considered: As long as you think of it just as a "Starter Kit", you simply don't need a large number of scenarios - it was about getting
started, and one does not need 100 scenarios to merely get
started. With the evolution into a product line, this perspective will change. And I believe so will be the number of published material in these times compared to the early days of SK.
von Marwitz